绘制简单图形——QPainter

QPainter实现鼠标绘制简单的图形（点、直线、圆、矩形）：

选择相应的图形按钮，鼠标点击左键开始绘制，松开鼠标完成绘制。

1.Mainwindow.h文件的相关声明

private:
    QPixmap pix;//画布
    QPoint lastpoint,endpoint;//存放鼠标坐标
    QCursor cursor; //设置默认光标
    QPushButton *line,*point,*circle,*rect;//图形选择按钮
    bool isline=false,ispoint=false,iscircle=false,isrect=false;//绘图标志
private slots:
    void initialization();//坐标初始化
     void Point();      
    void Line();
    void Circle();     
    void Rect();
protected:
    void paintEvent(QPaintEvent *);
    void mousePressEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);

2.Mainwindow.cpp文件

 pix=QPixmap(600,600);//设置画布大小
 pix.fill(Qt::lightGray);//画布颜色
 line=new QPushButton("线",this);                
 point=new QPushButton("点",this);
 circle=new QPushButton("圆",this);               
 rect=new QPushButton("矩形",this);
 point->setGeometry(700,90,50,30);          
 line->setGeometry(700,130,50,30);
 circle->setGeometry(700,170,50,30);       
 rect->setGeometry(700,210,50,30);
 connect(line,&QPushButton::clicked,this,&MainWindow2::Line);
 connect(point,&QPushButton::clicked,this,&MainWindow2::Point);
 connect(circle,SIGNAL(released()),this,SLOT(Circle()));
 connect(rect,SIGNAL(released()),this,SLOT(Rect()));

（1）paintEvent：

void MainWindow::paintEvent(QPaintEvent *)
{
    QPainter paint(&pix);
    QPen pen;
    pen.setColor(Qt::darkCyan);
    pen.setWidth(1);
    paint.setPen(pen);
    if(isline==true){
        if(endpoint.x()-lastpoint.x()!=0 || endpoint.y()-lastpoint.y()!=0)//排除绘制点的可能性
            paint.drawLine(lastpoint,endpoint);
        this->initialization();
    }
    if(ispoint==true){
        paint.drawPoint(lastpoint);      this->initialization();
    }
    if(iscircle==true){
        qreal r=sqrt(pow((endpoint.x()-lastpoint.x()),2)+pow((endpoint.y()-lastpoint.y()),2));
        paint.drawEllipse(lastpoint.x()-r,lastpoint.y()-r,2*r,2*r);
        this->initialization();
    }
    if(isrect==true){
        qreal width=endpoint.x()-lastpoint.x();
        qreal height=endpoint.y()-lastpoint.y();
        if(width!=0 && height!=0){
            paint.drawRect(lastpoint.x(),lastpoint.y(),width,height);
            this->initialization();
        }
    }
    QPainter painter(this);
    painter.drawPixmap(0,0,pix);
}

（2）按钮的点击事件：

void MainWindow::Line(){
    isline=true;
    ispoint=iscircle=isrect=false;
}
void MainWindow::Point(){
    ispoint=true;
    isline=iscircle=isrect=false;
}
void MainWindow::Circle(){
    iscircle=true;
    ispoint=isline=isrect=false;
}
void MainWindow::Rect(){
    isrect=true;
    ispoint=iscircle=isline=false;
}

（3）鼠标事件

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    if(isline==true||ispoint==true||iscircle==true||isrect==true){
        //按下左键，改变光标为十字形状
        QCursor c1;
        c1.setShape(Qt::CrossCursor);
        setCursor(c1);
        if(event->button()==Qt::LeftButton){
            lastpoint=event->pos();qDebug()<<lastpoint;
        }
        endpoint=lastpoint;
    }
}
void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
    if(event->button()&Qt::LeftButton){
        endpoint=event->pos();
        update();
    }
}
void MainWindow::mouseReleaseEvent(QMouseEvent *event)
{
    if(isline==true||ispoint==true||iscircle==true||isrect==true){
        if(event->button()==Qt::LeftButton){
            endpoint=event->pos();
            update();
        }
        setCursor(cursor);//完成绘制后，恢复光标形状
    }
}